Process of modifying the composition of the surface of a layer of plastic material



PROCESS OF MODIFYING THE COIVIPOSITION OF .THE SURFACE OF A LAYER OF PLASTIC MA- TERIAL Herman H. Duerr, Binghamton, N.Y., assignor toGeneral Aniline & Film Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed Mar. 17, 1958, Ser. No. 721,648

Claims. (Cl. 117--62) This invention relates to a process for modifying the composition of the surface of a layer of organic plastic material and thereby forming thereon an integral coating having a different chemical composition from the remainder of the layer of plastic material. More particularly, this invention relates to a process for preparing a saponified cellulose ester surface on a layer of cellulose ester which surface is suitable for use as a casting surface in the manufacture of film free from defects.

Heretofore a number of different processes have been used for casting film from cellulose ester solutions or dope. Certain of these prior art processes utilize a highly polished chromium-plated cylindrical drum having a diameter of -18 feet. Other processes utilize a polished metal band formed of metal sheets such as stainless steel or nickel, spliced together to form a continuous casting surface 50 to 100 feet long. These highly polished metal surfaces usually develop scratches, nicks and other indentations even under the most careful handling conditions. These defects in the casting surfaces are reflected in the finished films cast on such surfaces. Repairing these defects in the casting surfaces has been found to be a very costly and time-consuming procedure.

To overcome these difficulties in the use of highly polished metal casting surfaces, it has been the practice to select a metal for the casting drum or band which has good physical characteristics such as ductility, heat exchange and welding properties, and form the drum or band from such metal by welding together sections of the metal without polishing the surface of the metal. The desired mirror-like finish of the casting surface is then produced by coating the band with a suitable layer of organic plastic material such as a cellulose ester, which layer forms the casting surface upon which the film-forming solution is spread.

It is essential for the production of a good film that the casting surface of the band be insoluble in the solvent used to form the film-forming solution. Good films of cellulose ester cannot be produced by casting the filmforming solution upon a surface of cellulose ester. It has, therefore, been the practice in this art to partially hydrolyze or saponify the surface of the cellulose ester layer to form the actual casting surface.

Various methods of. saponification of the surface of the layer of cellulose ester have been suggested. One of these methods resides in treating the surface of the layer with sodium alcoholate using an apron to apply the alcoholate to the layer. Another method described in U.S.P. 2,622,278 to Eckler and Moeller resides in forming an organic solvent solution of ethyl cellulose and containing tree alkali, such as sodium or potassium hydroxide, and applying this solution to the cellulose ester layer. After the desired saponification has taken place, the ethyl cellulose film thus formed is stripped from the saponified surface produced.

A great deal of skill is required in the saponification step to produce a good casting surface, especially one that States Patent will swell uniformly, since any irregularities inthe casting surface Will leave their imprint in the film cast thereon. Uniform penetration of the cellulose ester layer by the hydrolyzing solution for the full surface area of the layer is an essential requirement for the production of a good casting surface. However, this uniform penetration of the hydrolyzing solution has not been achieved by the prior methods of saponification. In fact, uneven penetration of the hydrolyzing'solution into the cellulose ester layer usually occurs, especially during the starting and stopping of the application of the hydrolyzing solution, with the result that a considerable amount of expert hand operation is required to produce a casting surface which does not show a start or finish line.

It is an object of this invention to provide a process for treating the surface of a layer of organic plastic material with a reagent capable of chemically reacting therewith to produce on said layer an integral coating of a different chemical composition having uniform thickness and uniform properties throughout its extent.

It is a further object of this invention to provide a process for preparing a film casting surface which will be free from the foregoing and other disadvantages of the prior art processes hitherto employed for this purpose.

A still further object of the invention is to provide a process for preparing a filrncasting surface of a saponified cellulose ester which will swell uniformly throughout its extent when an alkali-containing, filmforming solution of a cellulose ester is spread thereon to produce a film free from defects.

Other objects and advantages of this invention will become apparent from the following detailed description thereof.

According to the present invention, a layer of a cellulose ester is first cast upon a metal drum or band in the usual manner and dried. A barrier layer of a filmforming material is then cast from a solution thereofon the dried cellulose ester on the drum or band. Any film-forming material which is soluble in solvents which do not attack the cellulose ester and which is inert to saponification reagents but permeable thereto may be used as this barrier layer. I have found that lower alkyl cellulose ethers, such as methyl cellulose, ethyl cellulose, propyl cellulose or the like, are especially suitable for this purpose. In order to increase the uniformity in the thickness of this barrier layer, it is preferable to form this layer by multiple applications of the solution of the cellulose ether rather than a single application,

Any alkaline reagent known in the art for de-esterifying cellulose esters may be used for my purpose. Such reagentsinclude potassium hydroxide, sodium hydroxide, potassium or sodium alcoholates, amines such as triethylamine or quaternary ammonium hydroxide.

The de-esterifying agent is applied over the barrier layer'in a solution of an organic film-forming substance such as ethyl cellulose, methyl cellulose, carboxymeth- 'yl cellulose, copolymers of styrene and maleic anhydride,

alkali-treated copolymers of vinyl ethers and maleic anhydride such as the copolymer of methylvinyl ether and maleic anhydride treated with ammonia vapors and the like. (See U.S.P. 2,699,392.) Preferably, I use a solution of ethyl cellulose in methanol containing an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide.

The de-esterifying film-forming solution is spread upon the barrier layer and the alkaline agent gradually penetrates through such layer from the upper to the lower surfaces thereof and reacts with the surface of the cellulose ester to hydrolyze or de-esterify the same. It is this gradual and controlled penetration of the saponifying reagent through the barrier layer that makes it possible to control the action of the saponifying reagent on the cellulose ester undercoat to obtain the desired depth of penetration thereof and to obtain a uniform distribution of the saponifying reagent over the entire surface of the cellulose ester undercoat. This is due to the fact that the saponifying reagent must diffuse first through the barrier layer before reaching and reacting with the cellulose ester layer. Accordingly, the start and stop marks, which are unavoidable with the prior art proce dures. are completely avoided. After the required depth of saponification of the cellulose ester has been obtained. the outer and barrier layers are stripped off and a mirrorlike casting surface of saponified cellulose acetate remains on the metal casting drum or band.

Any cellulose ester such as cellulose acetate. cellulose propionate, cellulose butyrate, cellulose acetate butyrate and the like in a suitable solvent therefor can be used to form the cellulose ester layer. Typically, a suitable cellulose ester layer can be formed from a solution comprising a low acetyl cellulose acetate (32% acetyl) dissolved in a solvent mixture of methylene chloride and methanol in a ratio of 9:1. The barrier layer may be of any desired thickness. I have found that a barrier layer of ethyl cellulose having a thickness of approxi mately 70 ma gives good results.

The following example of my process of producing a saponified cellulose ester film casting surface is given:

Example A layer of low acetyl cellulose acetate (32% acetyl) was formed on a conventional metal casting band by spreading a solution of the cellulose acetate in a solvent mixture of methylene chloride and methanol in the ratio of 9:1 and drying. A 15% solution of ethyl cellulose in methanol was then cast on the dried cellulose ester layer by spreading a number of thin layers of the solution on the cellulose acetate layer, one on top of the other, until a barrier layer of ethyl cellulose approximating 70 mi in thickness was formed. A dope containing 15 by weight of ethyl cellulose and .6% by weight of potassium hydroxide in methanol was then spread on the ethyl cellulose barrier layer and allowed to permeate therethrough and react with the surface of the cellulose acetate. After the reaction has progressed to the desired depth, the outer layer and barrier layer of ethyl cellulose were stripped from the cellulose acetate layer exposing a mirror-like surface of saponified cellulose acetate free of flaws and of uniform properties throughout its area.

It is to be understood that the foregoing detailed de scription is given merely by way of illustration and that many modifications may be made therein within the scope of the appended claims.

-I claim:

1. A process for chemically modifying the surface of a layer of organic plastic material, casting a solution of an organic plastic material in a volatile solvent on a supporting surface and drying to form a layer of said plastic material on said surface, forming a barrier layer on the layer of the organic plastic material by casting a solution comprising a lower alkyl cellulose ether dissolved in a volatile organic solvent which does not attack the first-mentioned organic plastic material, and drying to produce a uniform layer on said layer of plastic material, flowing a film-forming solution comprising an organic plastic material dissolved in a volatile organic solvent which does not attack the first mentioned organic plastic material and containing a reagent capable of chemically reacting with the first mentioned organic plastic material but inert to the second mentioned organic plastic material onto the barrier layer, whereby the said reagent permeates through the barrier layer and reacts with the first mentioned plastic material at the surface thereof and thereafter stripping from said first layer the layers cast thereover.

2. A process as recited in claim 1 wherein the cellulose ether is ethyl cellulose and said solvent is methyl alcohol.

3. A process for forming a film casting surface of saponified cellulose ester which comprises casting a solution of a cellulose ester on a metal support and drying to form a layer of the cellulose ester on the support, casting a solution of a lower alkyl cellulose ether in a volatile organic solvent therefor which does not attack the cellulose ester, and drying to form on said cellulose layer a barrier layer of a lower alkyl cellulose ether, flowing onto said barrier layer a film-forming solution comprising an organic plastic material dissolved in a volatile organic solvent therefor which does not attack the cellulose ester and containing a saponifying agent, whereby said saponifying agent permeates through the barrier layer and reacts with the surface of the cellulose ester layer to de-esterify the same, and stripping the outer layers to expose the saponified surface of said cellulose ester layer for use as a casting surface.

4. A process as recited in claim 3 wherein said cellulose ether is ethyl cellulose and said volatile organic solvent therefor is methyl alcohol.

5. A process as recited in claim 3 wherein said filmforming solution containing a saponifying agent comprises a solution of ethyl cellulose in methanol containing an alkali metal hydroxide.

6. A process as recited in claim 5 wherein said alkali metal hydroxide is potassium hydroxide.

7. A process as recited in claim 3 wherein said barrier layer has a thickness of my.

8. A process as recited in claim 3 wherein said metal support is a drum.

9. A process as recited in claim 3 wherein said metal support is in the form of a band.

10. A process as recited in claim 3 wherein the barrier layer is formed by casting a plurality of layers of the lower alkyl cellulose ether solution on said cellulose ester layer.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No. 2,974,061 March 7, 1961 Herman Hz. Duerr It is hereby certified'that error appears in the above numbered pat tters Patent should read as ent requiring correction and that the said Le corrected below after "cellulose second occurrence Comm 4 line 25, insert ester v Signed and sealed. this 25th day of July 1961;

I (SEAL) Attest:

DAVID L. LADD ERNEST W. SWIDER Attesting Officer Commissioner of Patents 

1. A PROCESS FOR CHEMICALLY MODIFYING THE SURFACE OF A LAYER OF ORGANIC PLASTIC MATERIAL, CASTING A SOLUTION OF AN ORGANIC PLASTIC MATERIAL IN A VOLATILE SOLVENT ON A SUPPORTING SURFACE AND DRYING TO FORM A LAYER OF SAID PLASTIC MATERIAL ON SAID SURFACE, FORMING A BARRIER LAYER ON THE LAYER OF THE ORGANIC PLASTIC MATERIAL BY CASTING A SOLUTION COMPRISING A LOWER ALKYL CELLULOSE ETHER DISSOLVED IN A VOLATILE ORGANIC SOLVENT WHICH DOES NOT ATTACK THE FIRST-MENTIONED ORGANIC PLASTIC MATERIAL, AND DRYING TO PRODUCE A UNIFORM LAYER N SAID LAYER OF PLASTIC MATERIAL, FLOWING A FILM-FORMING SOLUTION COMPRISING AN ORGANIC PLASTIC MATERIAL DISSOLVED IN A VOLATILE ORGANIC SOLVENT WHICH DOES NOT ATTACK THE FIRST MENTIONED ORGANIC PLASTIC MATERIAL AND CONTAINING A REAGENT CAPABLE OF CHEMICALLY REACTING WITH THE FIRST MENTIONED ORGANIC PLASTIC MATERIAL BUT INERT TO THE SECOND MENTIONED ORGANICV PLASTIC MATERIAL ONTO THE BARRIER LAYER, WHEREBY THE SAID REAGENT PERMEATES THROUGH THE BARRIER LAYER AND REACTS WITH THE FIRST MENTIONED PLASTIC MATERIAL AT THE SURFACE THEREOF AND THEREAFTER STRIPPING FROM SAID FIRST LAYER THE LAYERS CAST THEREOVER. 